It’s make or break time for Comet c/2012 S1 (ISON), a ball of ice hurtling toward the inner solar system that will make its closest approach to the sun this month. Whether ISON will flare into a “great comet” or fizzle out is still an open question, but scientists say either way, ISON offers an unprecedented opportunity to understand the ingredients and history of the solar system.
Comet ISON was discovered in September 2012 by two Russian astronomers using telescopes in the International Scientific Optical Network (ISON). The comet, made mostly of water and carbon dioxide ice, has been slowly making its way toward the sun from the Oort Cloud, the roughly spherical cloud of comets thought to extend about a light-year from the sun, about a third of the way to the nearest star. The comet “is going from the absolute coldest place in the solar system to the absolute hottest,” says Matthew Knight, an astronomer at Lowell Observatory in Flagstaff, Ariz. The ISON comet, known as sungrazer, will make its nearest pass to the sun on November 28, Thanksgiving Day, flying close to the surface. “We’ve never had a comet that seems to come directly from the Oort Cloud, on its first passage to the inner solar system in four billion years, all the way to within three solar radii of the solar surface,” says astronomer Michael Kelley of the University of Maryland, College Park. Kelley has been part of several campaigns to image ISON using telescopes in Hawaii, Arizona and the Canary Islands.
The exceptional opportunity has galvanized astronomers. Dozens of telescopes on the ground, in space, and on sounding rockets and high-altitude balloons are being trained on the comet as it approaches its showdown with the sun. They are tracking the comet in the visible, infrared, radio, x-ray and gamma-ray bands. Even satellites and rovers based at Mars and Mercury as well as spacecraft orbiting the sun have been enlisted to help. The project is being organized by the NASA Comet ISON Observing Campaign, which keeps a calendar view of all planned ISON observations.
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October 9 photographs from the Hubble Space Telescope and other measurements suggest ISON’s core is still intact and the comet is brightening somewhat. This news has heartened some who are still hoping the comet may put on a good show for naked-eye observers after its closest solar approach, or perihelion. “It’s still brightening but it has not been brightening as rapidly as we would have hoped,” Knight says. The object is currently about 10 times fainter than the unaided human eye can see, and just on the edge of being visible through ordinary binoculars. Just how bright it will get largely depends on whether ISON withstands its encounter with the sun or breaks up under the intense solar heat and tidal forces. If it survives, ISON is likely to flare much brighter than it is now as it curves back around the sun, and will enter a region of the sky that makes it prime viewing. “The next few weeks are going to be a very interesting time,” says Carey Lisse of the Johns Hopkins University Applied Physics Laboratory. Comet watchers are abuzz over some predictions that ISON is as good as dead, but observers in other camps insist the object is stable and looks promising.
Part of the reason ISON is so unpredictable is because it is the first of its kind. “It’s this unique combination where it’s something from the Oort Cloud that’s never experienced this heat before, and then it’s coming into this extreme environment where the change in temperature and gravitational pull makes it hard for us to predict what’s going to happen,” Knight says. Whether or not ISON becomes the “comet of the century,” as some have forecasted, it will almost certainly be useful scientifically. The Oort Cloud is made of the solar system’s leftover ingredients. “Once you have these bodies, you know how to build planets,” Lisse says. When ISON nears the sun, some scientists suspect its top layer will be dissolved, revealing a pristine underbelly—a window to the comet’s core. And ISON’s encounter with the sun could also serve as a probe of the solar atmosphere, a chance to test the strength of its magnetic field and solar wind. “It’s such a unique appearance that even if it disappoints from the ‘ooh and ah’ standpoint,” Knight says, “I think the science is going to be really interesting.”